It is desirable in MOS integrated circuits to minimize the chip area required to perform a given function. In this regard, it is generally advantageous to reduce the total amount of on-chip capacitance.
A circuit which typically utilizes several capacitors and which is conveniently embodied in an MOS integrated circuit is the switched-capacitor filter. The switched-capacitor filter is a type of analog filter wherein resistors are replaced by combinations of capacitors and switches. Various configurations for these circuits have heretofore been developed. In such circuits, it is the usual case that the filters' response characteristics are determined by capacitor ratios rather than by absolute capacitance values. It is also the usual case for these circuits to include differential amplifiers and for the switches of the circuit to be implemented with MOS enhancement-mode transistors.
Included among the various switched-capacitor filter circuits that have previously been developed, is one called a biquad which includes two differential amplifiers connected as integrators. The biquad switched-capacitor filter circuit typically requires the use of at least five switched capacitors. The values of those capacitors together with the choice of one of various possible arrangements of the capacitors determines the type of filter (high pass, low pass, or band pass, for example) and other characteristics of the filter.
For a fixed, specific filter application, appropriate capacitance values can be calculated and the circuit can be implemented with fixed capacitances. For greater versatility, though, it has heretofore been demonstrated that the response of a switched-capacitor filter can be dynamically varied by utilizing variable capacitors in the circuit.
An extremely versatile, dynamically changeable filter would result if every switched-capacitor in a conventional biquad filter was variable or programmable over a wide range of capacitance values. Considerable chip area, though, would likely be required to implement such a circuit in an MOS integrated circuit. Moreover, because it is frequently desirable to embody several filters in an integrated circuit (to make high-order filters, for example), the chip area problem could be compounded.
A technique heretofore known for reducing the number of components required for a bank of switched-capacitor filters is the sharing of an amplifier among a number of filters in a time multiplexed manner, thereby reducing the number of amplifiers required and reducing chip area. That approach, however, does not reduce the number of capacitors or the total amount of capacitance required for a switched-capacitor filter, nor does that approach provide a versatile, dynamically changeable filter.
In accordance with the foregoing, a need exists for a method for reducing the number of capacitive elements required in electronic circuits, and especially in switched-capacitor filter circuits. A need also exists for versatile, dynamically changeable circuitry which can conveniently be embodied in an MOS integrated circuit, and wherein the required number of capacitive elements is minimized.